Citations

Abstract Abstract 1586 Transformation growth factor (TGF-β) is a highly pleiotropic cytokine critical to a variety of cellular events such as cell differentiation and apoptosis. TGF-β is synthesized as a prepro-TGF-β precursor and secreted after being processed in Golgi apparatus as a latent form that non-covalently combines both TGF-β and latency-associated protein (LAP). Our previous work in B-cell NHL has shown that the intratumoral T cell composition results in the establishment of a profoundly inhibitory tumor microenvironment. However, the underlying mechanism is only partially understood. In this study, using patient specimens and lymphoma cell lines, we evaluated the role of TGF-β in the tumor microenvironment and determined the effect of TGF-β on the generation of intratumoral TH1, TH17 and Treg cells in B-cell NHL. First, we determined expression of TGF-β and found that a latent form of TGF-β was specifically expressed on the surface of CD19+ B cells, but not on other types of cells from B-cell lymphoma biopsy specimens. By screening cell lines, we found that latent TGF-β was also expressed on the surface of lymphoma cell lines, confirming the finding. Second, we tested whether surface expression by lymphoma cells led to the secretion of TGF-β in culture medium. Using an ELISA assay, we detected variable levels of latent TGF-β in the culture medium of primary malignant B cells (median 100 pg/ml per million cells, range: undetectable −229 pg/ml, n=7). Similarly, lymphoma cell lines secreted variable amounts of TGF-β from undetectable to 200 pg/ml per million cells. Next, we determined the effect of TGF-b on intratumoral T cell proliferation and differentiation. As expected, exogenous addition of TGF-β inhibited the proliferation of T cells. Notably, the proliferation of intratumoral T cells was significantly reduced when co-cultured with lymphoma cells bearing an active form of TGF-β compared to that with lymphoma cells without TGF-β. Using flow cytometry, we showed that the addition of exogenous TGF-β enhanced Foxp3 expression in activated CD4+, CD4+CD45RA+ or CD4+CD45RO+ intratumoral T cells, suggesting that TGF-β promotes the generation of Treg cells in tumor microenvironment. In contrast, TGF-β suppressed the expression of IFN-γ in activated CD4+ T cells and inhibited the up-regulation of IL-12 and IL-23-induced IFN-γ expression in CD4+ cells, indicating that TGF-β suppresses the generation of TH1 cells. TGF-β alone slightly inhibited IL-17 expression in CD4+ T cells; however, TGF-β, in the presence of IL-6 and IL-23, up-regulated IL-17 expression in CD4+ T cells, suggesting proinflammatory cytokines are able to reverse the suppression induced by TGF-β. These results suggest that TGF-β controls the generation of TH1, TH17 and Treg cells contributing to the imbalance of effector TH cells and inhibitory Treg cells in the tumor microenvironment of B-cell NHL. Since malignant B-cells produce TGF-β, these results further support the important role of malignant B cells in the regulation of intratumoral T cell differentiation and the host immune response. Disclosures: No relevant conflicts of interest to declare.

Abstract While extensive, the genetic studies of DLBCL tumors to date have primarily focused on somatic mutations that are associated with development of lymphoma, i.e., driver mutations. Identification of key genetic events that impact and/or predict response to therapy and clinical outcome of lymphoma patients has not been accomplished. In an exploratory study, we used whole-exome sequencing to identify novel biomarkers that can predict which patients have an aggressive form of DLBCL, and therefore would likely benefit from a different and more aggressive treatment plan. Using whole-exome sequencing data from 54 newly diagnosed DLBCL patients, we evaluated the association of somatic coding single nucleotide (cSNV) and copy number (CNV) variants with aggressive DLBCL. All patients were treated with R-CHOP or immunochemotherapy, and disease aggressiveness was based on relapse, with patients classified as having aggressive disease (AD) (n=13, defined as treatment failure, relapse, or progression within 24 months of diagnosis) versus non-aggressive disease (n=41, defined as achieving at least 24 months of relapse-free survival). An in-house algorithm, patternCNV was used to detect somatic CNVs. Logistic regression was used to assess statistical significance. In the cSNV analysis, 24 genes were found to be significantly (p ≤ 0.05) associated with AD. Of these genes, CIITA (p=0.01) was previously identified as a mutational target in DLBCL and is a recurrent gene fusion partner in lymphoid cancers. Metacore analyses showed that seven of these genes, including CIITA, can be placed in the same regulatory network centered around CREB1. Interestingly, the CREB binding protein (CREBBP) is a known target of pathogenic mutations in DLBCL. In the CNV analyses, we identified 245 gene amplifications and 209 gene deletions associated with AD (p ≤ 0.05). Deleted genes were localized in three major chromosomal regions, 6p22.3-6p22.1 (26 genes), 6q13-6q16.3 (31 genes), and 6q21-6q24.2 (86 genes). In the CNV amplification analysis, the genes were localized in two major chromosomal regions, 3q12.3-3q29 (90 genes) and 19q13.12-19q13.43 (122 genes). We also quantified the association of each regional deletion/amplification with patient outcome. As expected, each deleted region was univariately associated with time to relapse: 6p22.3-6p22.1 (HR = 7.19, 95% CI: 2.66-19.46, p=1.33x10-5), 6q13-6q16.3 (HR = 4.44, 95% CI: 1.74-11.32, p=6.84x10-4), and 6q21-6q24.2 (HR = 5.42, 95% CI: 2.12-13.87, p=9.23x10-5) as well as the two amplification regions: 3q12.3-3q29 (HR = 4.60, 95% CI: 1.49-14.24, p=4.27x10-3) and 19q13.12 -19q13 (HR = 13.79, 95% CI: 4.39-43.34, p=2.81x10-8). Deletions at 6q21-6q24.2 have been previously reported in DLBCL and two tumor suppressor genes associated with lymphoma pathogenesis are localized in the region, TNFAIP3 (p=0.14) and PRDM1 (p=0.02), the latter of which was significantly associated with AD. However, the genes in the 6p21 locus that were most significant (p=0.002) were SLC22A16, GPR6, SOBP, MATTL24, DDO, and REV3L. SLC22A16, which also had cSNVs in two tumors, is an organic cation transporter that has been shown to transport chemotherapeutic drugs including doxorubicin. Successful drug response has been correlated with the level of activity and expression of this transporter in tumor cells. To determine if expression of SLC22A16 would have an impact on doxorubicin transport, and may therefore impact response to R-CHOP we overexpressed either an empty vector control or SLC22A16 in OCI-Ly7 DLBCL cells, which do not express endogenous SLC22A16 mRNA. We found that OCI-Ly7 cells expressing SLC22A16 have increased 14C-doxorubicin uptake and are more sensitive to doxorubicin-induced cell death when compared to the empty vector control cells. These data suggest that loss of SLC22A16 expression through gene deletion could impact the ability of DLBCL cells to respond to therapy and indicates an aggressive form of the disease. In summary, our data are the first to use whole exome sequencing to identify genetic variants associated with aggressive DLBCL, which will require replication in additional samples. Nevertheless, our data highlight the role for somatic CNVs in patient outcome and we biologically validate the potential impact of deletions in SLC22A16 at 6p21. Disclosures: No relevant conflicts of interest to declare.

Abstract Background: Aberrant B-cell receptor (BCR) and MYD88 signaling cooperatively drive Waldenstrom macroglobulinemia (WM) tumor cell survival. To sustain BCR-driven growth and maintain optimal cellular homeostasis, WM cells upregulate proteasome function. We have recently described the role of two distinct 19S proteasome-associated deubiquitinating enzymes (DUBs), USP14 and UCHL5, in maintaining WM-cell viability. Pharmacologic inhibition with b-AP15 (commercial compound) results in WM-cell death and accumulation of high molecular weight ubiquitinated protein conjugates; an event that is phenocopied by siRNA knockdown of USP14 and UCHL5. VLX1570 is a lead-optimized clinical grade analog of b-AP15 with enhanced solubility, stability and target residence time. We investigated the effect of VLX1570 on BCR components, WM cell survival proteins and its in vivo activity in a human xenograft model of WM. Materials: VLX1570 was obtained from Vivolux AB and bortezomib from Sellekchem. Human WM cell lines (BCWM.1, MWCL-1 and RPCI-WM1) and their drug-resistant derivatives were used (total n=12). Bone marrow aspirates from WM-patients and from healthy donors were obtained from the Mayo Clinic biospecimens core. PBMCs from healthy donors were used as comparators where needed. NOD/SCID mice were obtained from Taconic Labs and used for profiling the in vivo properties of VLX1570. Results: We first examined for USP14/UCHL5 protein expression by immunohistochemical analysis in primary tumor tissue from WM patients and compared it to healthy donor cells. WM patient-derived cells exhibited greater staining intensity of both DUBs. Preferential binding of VLX1570 to USP14 and UCHL5 in WM cells was probed in a WM cell using an enzyme-based competition assay (HA-Ub-Vs), which demonstrated time-dependent decrease in labeled USP14 and UCHL5 after treatment with VLX1570. In drug-resistant (bortezomib or ibrutinib-resistant) WM cells, VLX1570 induced >50% tumor-specific apoptosis and displayed synergistic activity with ibrutinib (median CI: 0.5). In addition to ER stress markers, we had previously observed that gene expression of BCR-signalosome components were reduced in VLX1570-treated WM cells (NanoString platform) and this was substantiated at the protein level where p-SYK, SYK and BTK decreased in a time-dependent manner. BCR end-effectors were probed in response to VLX1570 and a decrease in nuclear translocation of NFkB (p65) was noted. USP14 has been shown to control the availability of the chemokine receptor CXCR4, which regulates homing of WM cells to bone marrow. After VLX1570 treatment, we noted a significant decrease in CXCR4 surface receptor expression in all WM cell lines tested and primary WM cells. Lastly, to determine the in vivo anti-WM effects of VLX1570, we injected NOD/SCID mice (xenotransplanted with RPCI-WM1-Luc cells) with VLX1570 (I.P. route, 4.4mg/kg every other day) and measured tumor burden along with pharmacodynamic indicators. VLX1570-treated mice (n=7) demonstrated significantly lower tumor volumes by caliper measurement and bioluminescent signal (p=0.01 - 0.003) as well as reduced serum human-IgM, compared to vehicle-treated mice (n=7). Importantly, mice treated with VLX1570 survived longer vs. vehicle-treated mice (median survival, 62 vs. 44 days, respectively, p=0.0008). No unforeseen toxicities were noted and weight of the mice in vehicle vs. VLX1570-treated mice did not significantly change. IHC analysis in mice tumor tissues demonstrated reduced CXCR4, human-IgM, Ki67 and increased cleaved caspase-3 and ubiquitinated (k48-linked) protein level, the latter serving as a more specific indicator of USP14/UCHL5 inhibition. Conclusions: We provide substantial in vitro and in vivo evidence on the anti-WM effects of a novel first-in-class clinical grade USP14/UCHL5 inhibitor, VLX1570. In addition to anticipated induction of ER stress machinery, we describe for the first time VLX1570-mediated modulation of BCR components and WM cell survival proteins as well as demonstrate its robust activity in an aggressive human xenograft model of WM. These data provide the rationale for a clinical trial of single agent VLX1570 in patients with symptomatic WM. Disclosures Martin: Janssen: Consultancy, Honoraria; Acerta: Consultancy; Gilead: Consultancy; Celgene: Consultancy; Novartis: Consultancy; Bayer: Consultancy. Gullbo:Vivolux AB: Other: Shareholder. Linder:Vivolux AB: Other: Shareholder.

Abstract T-cell exhaustion plays an important role in attenuating the function of immune cells in B-cell non-Hodgkin's lymphoma (NHL) and PD-1 expression is typically used to identify exhausted T-cells. We have however previously shown that not all PD-1+ cells are exhausted and that PD-1 is differentially expressed on two distinct T-cell subpopulations, with high expression on T follicular helper cells and dim expression on exhausted T cells. Other markers are therefore needed to more clearly identify exhausted intratumoral T cells. To further define exhaustion of intratumoral T cells, we determined the co-expression, regulation and function of PD-1, TIM-3 and LAG-3 on CD4+ or CD8+ T cells by flow cytometry. Using biopsy specimens from follicular B-cell NHL, we found that the percentages of PD-1+ and TIM-3+ T cells were 53.1% (range: 17.2-81.2%, n=32) and 34.5% (range: 14.9-62.6%, n=34) in CD4+ T cells and 46.8% (range: 12.8-81.7%, n=32) and 40.4% (range: 15.0-78.4%, n=34) in CD8+ T cells, respectively. We observed that TIM-3 was predominantly expressed on PD-1dim T cells and TIM-3+ cells accounted for 40% of CD4+ PD-1dim or 45% of CD8+ PD-1dim T cells. Similarly, LAG-3 was variably expressed on intratumoral T cells from B-cell NHL. A median of 9.54% (range: 3.01-15.46, n=6) of CD4+ or 20.48% (7.93-33.9, n=8) of CD8+ T cells express LAG-3. We found that LAG-3+ T cells almost exclusively came from PD-1+ TIM-3+ cells, forming a defined population of intratumoral PD-1+ TIM-3+ LAG-3+ CD4+ or CD8+ T cells. While the majority of LAG-3+ T cells were effector memory T cells (CD45RA- CCR7-), some LAG-3-expressing T cells displayed a phenotype of terminally-differentiated T cells (CD45RA+ CCR7-). Functionally, the intratumoral TIM-3+ LAG-3+ T cells exhibited reduced capacity to produce cytokines (IL-2, IFN-γ) and granules (perforin, granzyme B). Similar to TIM-3, LAG-3 expression was strongly up-regulated on CD4+ or CD8+ T cells by IL-12, a cytokine that has been shown to induce T-cell exhaustion. Interestingly, we observed that while expression of TIM-3 on CD8+ T cells was upregulated by IL-12 at an early time point (day 1), LAG-3 was only induced after TIM-3 up-regulation (day 3) and almost exclusively on TIM-3+ T cells. Furthermore, we found that blockade of both TIM-3 and LAG-3 signaling was able to reverse the exhausted phenotype of CD8+ T cells resulting in increased IFN-γ and IL-2 production. This effect was further enhanced when CD8+ T cells were treated with both anti-TIM-3 and anti-LAG-3 Abs. Taken together, these results suggest that PD-1, TIM-3 and LAG-3 were involved in the induction of exhaustion of T cells in B-cell NHL. We find that PD-1, TIM-3 and LAG-3 are expressed on the same T cells and that blocking TIM-3 and LAG-3 can reverse T-cell exhaustion signaling. These results suggest that PD-1, TIM-3 and LAG-3 play a synergistic role in the development of T cell exhaustion in NHL. Disclosures No relevant conflicts of interest to declare.

Abstract Recent next generation sequencing of paired normal and tumor lymphoma tissue highlights a significant role for the MYD88L265P mutations in Waldenstrom Macroglobulinemia (WM) and diffuse large B-cell lymphoma (DLBCL). In recent work we have found that MYD88L265P signaling is constitutively active in both WM and DLBCL cells leading to heightened MYD88L265P, IRAK and TRAF6 oligomerization and TAK1 and NF-kB activation. However, little else is know about the full impact of the mutation, especially in combination with other genetic events. Recent mouse models suggest that MYD88L265P alone may not be sufficient to induce tumor formation and that the pathway requires additional genetic hits. Interplay between MYD88L265P and other oncogenic events are further supported by the fact that TNFAIP3 (A20) inactivation often accompanies MYD88L265P. These data, combined with the finding that loss of TNFAIP3 promotes accumulation of MYD88L265PB-cells in a mouse model, suggest a potential relationship between these genetic events, and more importantly, a possible clinical impact. The aim of this study was to generate a human cell line model which mimics the effect of MYD88L265P andTNFAIP3 loss to further understand the cellular consequence of these genomic alterations in lymphoma. To mimic the TNFAIP3 loss in combination with the MYD88L265P mutation,we used the WM cell line MWCL1, which harbors a heterozygous mutation in MYD88. For the TNFAIP3 specific knock out, we used the transcription activator-like effector nucleases (TALENs) genome editing approach to induce a double stranded break in exon 5 of the TNFAIP3 gene. TALENs were expressed in MWCL1 cells by transient transfection, and single-cell clones were isolated. Sanger sequencing of the TNFAIP3 gene was performed to identify clones that carried the deletion and all clones were screened by Western blot to confirm knock down of A20. We successfully generated a panel of MWCL1 cell lines that harbor a heterozygous deletion in the TNFAIP3 gene. Western blot analysis confirm a 2.4 fold reduction in A20 expression in the heterozygous cell line (MWCL1A20-/A20+), Because A20 has been shown the be a central negative regulator of NF-κB activation, we first measured the impact of A20 loss on phosphorylation of NF-κB p65 and found that MWCL1A20-/A20+ cells had higher base line levels of pNF-κB compared to the wild type (WT) cell line (1.77 fold). Furthermore, time course stimulation experiments with the TLR ligand lipopolysaccharide (LPS) showed that MWCL1A20-/A20+ cells had a higher amount of pNF-κB after 30 (3 fold) and 60 (1.5 fold) minutes of stimulation. We next looked at expression of NF-κB target genes by quantitative real time PCR and found that IL-6 (2.5 fold; p≤ 0.05) and CXCL10 (IP10) (4 fold; p≤ 0.05) were upregulated in MWCL1A20-/A20+cells compared to WT. In summary, we have established a new WM cell line model which mimics the effect of the MYD88L265P mutation in combination with a heterozygous loss of the TNFAIP3 gene. We show that loss of TNFAIP3 results in a higher baseline and TLR-stimulated phosphorylation of NF-κB as well as in higher expression IL-6 and IP10. Overall, this cell line will be a useful tool to study biological consequences of TNFAIP3loss in WM patients as well as novel therapeutic strategies. Disclosures Ansell: BMS, Seattle Genetics, Merck, Celldex and Affimed: Research Funding.

Abstract Abstract 2702 Background: CXCR5 [chemokine (C-X-C motif) receptor 5; also known as Burkitt lymphoma receptor 1 (BCR1)] is expressed on mature B-cells, subsets of CD4+ and CD8+ T-cells, and skin-derived migratory dendritic cells. CXCL13 is the ligand for CXCR5, and together they are involved in guiding B-cells into the B-cell zones of secondary lymphoid organs as well as T-cell migration. CXCR5 is also expressed in malignant cells in NHL patients. Single nucleotide polymorphisms (SNPs) in CXCR5 have not been extensively studied, although two studies reported that SNPs from this chromosome 11 gene were associated with risk of primary biliary cirrhosis in a European population and with risk of NHL in an Asian population. This study evaluated the role of common germline genetic variation in the risk and prognosis of the common NHL subtypes of diffuse large B-cell lymphoma (DLBCL), follicular lymphoma (FL), mantle cell lymphoma (MCL), marginal zone lymphoma (MZL) and peripheral T-cell lymphoma (PTCL). Methods: Ten SNPs in CXCR5 were genotyped using a clinic-based study of 2694 NHL cases (including 586 DLBCL, 588 FL, 137 MCL, 230 MZL, and 158 PTCL) and 1521 controls enrolled from 2002–2009 in the Molecular Epidemiology Resource of the University of Iowa/Mayo Clinic Lymphoma SPORE. Clinical data were abstracted for all NHL patients and all were prospectively followed for disease progression, retreatment, transformation and death. SNPs were selected from HapMap using a standard tagging approach (r2≥0.80 and minor allele frequency (MAF) ≥0.05) and included SNPs 5kb upstream and downstream from the gene. Genotyping was conducted using a custom Illumina iSelect panel. All SNPs were in Hardy-Weinberg equilibrium in the control group. The most prevalent homozygous genotype was used as the reference group, and each SNP was modeled as having a log-additive effect in the regression model. For risk of NHL, odds ratios (ORs) and 95% confidence intervals were calculated using unconditional logistic regression, adjusted for age and gender. For prognosis, hazard ratios (HR) were calculated for both event-free (EFS) and overall (OS) survival using Cox proportional hazards regression, adjusting for age, treatment type, and IPI (DLBCL, PTCL), FLIPI (FL), or MIPI (MCL). To determine risk of FL transformation to DLBCL, a competing risk Cox model was used. A p<0.05 was declared as statistically significant. Results: Of the 10 CXCR5 tag SNPs in our study, 5 were associated with risk of NHL, with rs1790192 (MAF=0.39 among controls) having the strongest association (OR=1.19, 95%CI 1.08–1.30; p=0.0003). This SNP was strongly associated with FL (OR=1.44, 95%CI 1.25–1.66; p=3.1×10−7), and was also associated with DLBCL (OR=1.16, 95%CI 1.01–1.33; p=0.038) and PTCL (OR=1.29, 95%CI 1.02–1.64; p=0.037) but not MCL or MZL. For FL, none of the SNPs were associated with EFS or OS, although rs1790192 was associated with better outcome among FL patients that were observed (HR=0.64; 95%CI 0.47–0.87; p=0.0039). For DLBCL, none of the SNPs were associated with EFS, while three SNPs were associated with OS: rs523604 (HR=0.79; 95%CI 0.64–0.97; p=0.028), rs12363277 (HR=0.57; 95%CI 0.33–0.99; p=0.046), and rs3922 (HR=0.79; 95%CI 0.65–0.97; p=0.027). The latter SNP was also associated with risk of transformation from FL to DLBCL (HR=0.61; 95%CI 0.40–0.91; p=0.016). For MCL, MZL and PTCL, none of the SNPs were associated with either EFS or OS. Conclusions: Several germline CXCR5 polymorphisms were associated with NHL risk, particularly rs1790192 with risk of FL as well as DLBCL and PTCL. CXCR5 SNPs did not appear to be important in prognosis for most NHL subtypes, although rs1790192 was associated with a better outcome in untreated FL and rs3922 was associated with OS in DLBCL patients and with risk of FL transformation. These results provide additional evidence for a role of host genetic variation in CXCR5 in lymphomagenesis, mainly in risk of disease development, although there may also be a role for CXCR5 in FL and DLBCL prognosis. Replications of these findings in additional studies are needed. Disclosures: Ansell: Seattle Genetics, Inc.: Research Funding.

Abstract Abstract 2688 Members of the signal transducers and activators of transcription (STAT) family of proteins function as secondary messengers mediating cellular responses to various cytokines. Aberrant activation of STAT5 protein has been implicated in the pathogenesis of hematologic malignancies due to the ability of these transcription factors to regulate genes involved in cellular proliferation and survival. The two highly homologous transcription factors collectively known as STAT5, namely STAT5A and STAT5B, display both redundant and distinct physiologic functions in non-malignant B cells. Yet, despite evidence suggesting independent functions for STAT5A and STAT5B in the pathophysiology of solid tumors, the precise roles of these isoforms in hematologic malignancies such as Waldenstrom's macroglobulinemia (WM) are not known. Initial immunohistochemical staining of WM bone marrow biopsies (n=6) revealed higher levels of STAT5 phosphorylation compared to bone marrows from normal controls (n=6). Additionally, phosphorylated STAT5 was highly expressed in CD19/CD138+ sorted cells obtained from the bone marrow of patients with WM (average ΔMFI compared to isotype control, 4.64 +/− 1.7, n=6) as determined by FACS analysis. Furthermore, western blotting demonstrated baseline STAT5 phosphorylation in both WM and IgM-secreting cell lines (MWCL-1 and BCWM-1, respectfully). The expression of other phosphorylated STAT proteins (1, 3, 4, and 6) was not detected. To better understand the overall role of STAT5 biologically in WM, MWCL-1 and BCWM.1 cells were treated with a specific inhibitor of STAT5, N′-((4-Oxo-4H-chromen-3-yl)methylene) nicotinohydrazide. Following 72 hours in culture, this inhibitor significantly decreased IgM secretion at lower doses when compared to controls (10 μM, MWCL-1: 9413 +/− 511 ng/mL vs. 6166 +/− 160 ng/mL, p<0.001; BCWM.1: 13,380 +/− 393 ng/mL +/− 10970 +/− 240 ng/mL, p<0.001) and nearly abolished IgM secretion at higher doses. Significant effects on proliferation and viability were also observed, but only at higher concentrations of drug where the specificity toward STAT5 may have diminished. To characterize the roles of the individual STAT5 isoforms in WM, we have developed doxycycline-inducible STAT5A-shRNA and STAT5B-shRNA and used these to create stable knockdowns in the MWCL-1 cell line. Addition of doxycycline to these cell lines for 72h nearly abolishes STAT5A or STAT5B protein expression, respectively, without affecting expression of the other isoform, while no change in STAT5 expression is detected in cells transduced with the empty vector (EV) control. The functional effects of isoform-specific STAT5 inhibition on WM tumor biology were assessed through proliferation assays and ELISA to determine the utility of therapeutically targeting a single STAT5 isoform. No difference in proliferation was observed in the STAT5A- or STAT5B-shRNA cells following 72h induction with doxycycline as compared to the EV control cells. However, specific knockdown of STAT5A following doxycycline induction significantly decreased IgM secretion as compared to the EV control (EV, 91.8 +/− 10.2% of baseline secretion, STAT5A, 66.7 +/− 15.1% of baseline, p<0.05), whereas IgM secretion was decreased by knockdown of STAT5B but not significantly. Similarly, IL-6 secretion was only diminished in cells lacking STAT5A as compared to EV cells (EV, 94 +/− 14.16% of baseline secretion; STAT5A, 64% +/− 7.6% of baseline, p<0.001). Overall these initial data indicate a role for isoform-specific STAT5 activation in the maintenance of WM tumors, and future studies will further our understanding of the specific roles of STAT5A and STAT5B in WM tumors. Disclosures: Ansell: Seattle Genetics, Inc.: Research Funding.

Abstract Abstract 2699 Myeloid differentiation factor 88 (MYD88) is an adaptor protein that plays a key role in mediating innate immune response. Upon activation of toll-like (TLRs) or IL-1b receptors, MYD88 activates a signaling cascade that culminates in activation of NF-kB and expression of proinflammatory genes. Activating mutations in MYD88 have been reported in non-Hodgkin's lymphoma (NHL) tumors, including diffuse large B cell and marginal zones lymphoma. The most common MYD88 mutation described thus far is a single base pair mismatch resulting in an amino acid switch from lysine to proline at position 265 (MYD88L265P). MYD88L265P functions as a constitutively active MYD88 protein resulting in dysregulated NF-kB and STAT3 signaling. This mutation has recently been reported to be present in lymphomoplasmacytic lymphoma/Waldenstrom macroglobulinemia (LPL/WM) at a high frequency, ∼ 90% (Xu et al, ASH, #261, 2011). The discovery of MYD88L265P in a high percentage of LPL/WM tumors, and other subtypes of NHL, provides the basis for the identification and targeting of MYD88 dependent pathways. Additionally, the ability of this mutation to drive NF-kB-mediated cytokine expression combined with our finding that LPL/WM patients have elevated serum cytokines suggests that the MYD88 mutation may play a significant role in LPL/WM pathogenesis. We first wanted to confirm the incidence of the MYD88L265P mutation in LPL/WM and other subtypes of indolent lymphoma. Using Sanger sequencing we found that 57% (n=58) of LPL, 4% of MALT (n=23), 5% of nodal marginal zone (n=34), and 6% of splenic marginal zone (n=34) lymphomas harbor the MYD88L265P mutation, with all but 1 case being heterozygous. When we specifically looked at the LPL cases with WM, we found that 70% of WM tumors have the MYD88L265P mutation. In a validation cohort of WM (n=41), we found a similar mutation rate of 68%. Additionally, the WM cell line MWCL1 and the IgM secreting cell line BCWM.1 also harbor the MYD88L265P mutation. The prevalence of the MYD88L265P mutation in NHL, specifically WM and LPL, suggests that it may be a novel driver of lymphoma pathogenesis and a potential therapeutic target. However, it is not yet clear if this mutation correlates with activation of signaling pathways, or cell growth and survival in LPL/WM. To address these questions we first explored the possibility that the MYD88 signaling pathway is constitutively active in cells that express MYD88L265P. Upon activation of TLRs or IL-1b receptors, MYD88 forms a homodiamer and recruits IRAK1/4 and TRAF6 into a complex resulting in association and phosphorylation of TAK1 followed by activation of NF-kB. Using three cell lines that express MYD88L265P (BCWM, MWCL1, and OCI-Ly3), we looked for complex formation of MYD88, IRAK1/4, TRAF6, and TAK1. Immunoprecipitation of either endogenous IRAK1 or TAK1 revealed constitutive association with both TRAF6 and MYD88. We also detected phosphorylation of TAK1, and taken together, these data suggest that the MYD88 pathway is activated in cells that harbor the MYD88L265P mutation. We next wanted to confirm the significance of the MYD88 pathway on lymphoma cell growth. Using our cell lines we tested the effect of IRAK1/4, TAK1, and NF-kB inhibitors on cell proliferation. All MYD88L265P expressing cell lines were sensitive to the TAK1 and NF-kB inhibitors in a dose dependent manner (0–10 uM), however, we only saw inhibition of cell proliferation with the IRAK1/4 inhibitor at the highest dose (10 uM) in the BCWM cell line. These data suggest that cell proliferation is dependent on the MYD88 signaling pathway, with some variation being detected between cell line models and inhibitors. Because the MYD88 pathway has also been shown to drive cytokine secretion we next tested the impact of the IRAK1/4, TAK1, and NF-kB inhibitors on expression of IL-10. Using drug doses that were shown to inhibit cell proliferation, but not impact survival, we found that all three inhibitors reduced the level of IL-10 secreted by each of cell lines. In summary, our data confirm the high incidence on the MYD88L265P mutation in LPL/WM, we find that the MYD88 signaling pathway is constitutively activated in cells the harbor the mutation, and we show that cell proliferation and cytokine secretion are dependent on MYD88 signaling targets. These studies highlight the biologic significance of the MYD88L265P mutation in LPL/WM and suggest that the MYD88 signaling pathway may be a novel therapeutic target. Disclosures: No relevant conflicts of interest to declare.

Abstract Abstract 468 The requirement for BAFF and BAFF-R in normal human and murine B cells is well studied, but there is also significant evidence to suggest that BAFF plays an important role in malignant B cell proliferation and survival. Serum BAFF levels are elevated in patients with non-Hodgkin lymphoma (NHL) and high BAFF levels correlate with aggressive disease and a poor response to therapy. There is also increasing genetic evidence suggesting an association between the development of human disease and genetic variation in genes encoding BAFF and its receptors. Mutations in TNFRSF13B (TACI) were identified in patients with familial common variable immunodeficiency (CVID) and IgA deficiency and we have found that single nucleotide polymorphisms (SNP) in TNFSF13B (BAFF) are associated with elevated BAFF levels and risk for developing NHL. To build upon these findings we sequenced BAFF and its receptors; TNFSF13B, TNFRSF13B, TNFRSF17(BCMA), and TNFRSF13C (BAFF-R) in NHL patients to identify novel genetic variants that may be associated with NHL risk. Among 40 individual samples (20 controls and 20 follicular lymphoma (FL) cases) that were bi-directionally sequenced we identified a heterozygous cytosine to thymidine transition in 1 patient specimen at position 475 (C475T) of TNFRSF13C. The C475T transition encodes a missense substitution of tyrosine for histidine in codon 159 (H159Y) in the highly conserved cytoplasmic tail of BAFF-R, adjacent to the TRAF3 binding motif PVPAT. We next expanded our analysis of BAFF-R H159Y and analyzed NHL tumor biopsies for the presence of the mutation. 4/41 (10%) follicular lymphomas (FL), 2/42 (5%) diffuse large B cell lymphomas, 1/22 (5%) lymphoplasmacytic lymphomas (LPL), and 1/24 (4%) mucosal associate lymphoid tissue lymphomas carried the heterozygous mutation. The BAFF-R H159Y mutation was not detected in any of the normal control DNA from healthy donors (n=100). Given its close proximity to the TRAF3 binding site in the cytoplasmic domain of BAFF-R we first wanted to determine if the H159Y mutation altered BAFF induced signaling. We generated cell lines that express HA-tagged wildtype BAFF-R, BAFF-R with the H159Y mutation, or BAFF-R with an ablated TRAF3 binding site as a negative control. Analysis of cells expressing H159Y BAFF-R demonstrates that this mutation results in increased BAFF-R-mediated NFκB1 and NF-κB2 activation. The enhanced signal activated by BAFF-R H159Y is coupled with a several fold increase in TRAF3, TRAF2, and TRAF6 recruitment to BAFF-R and increased IgM production. We further demonstrate that recruitment of TRAF6 to BAFF-R is not unique to the mutant H159Y BAFF-R, but is also an important and necessary feature of BAFF-R signaling in normal B cells. Collectively, our data identify a novel lymphoma-associated mutation in BAFF-R and describe exciting new aspects of BAFF-R signaling that are important for understanding normal B cell homeostasis and function, as well as pathogenic BAFF-R contributions to human disease. Disclosures: No relevant conflicts of interest to declare.

Abstract Waldenstrom’s macroglobulinemia is a serious and frequently fatal illness, however many of the mechanisms leading to this disease are not yet known. It is clear, however, that there is dysregulation of the balance between cell proliferation and programmed cell death. BLyS (B-lymphocyte stimulator) is a newly identified TNF family member expressed by monocytes, macrophages, and dendritic cells. BLyS has been shown to be critical for maintenance of normal B cell development and homeostasis, and has been found to stimulate lymphocyte growth. BLyS is overexpressed in a variety of B-cell malignancies and has been shown to inhibit apoptosis in malignant B-cells. Studies of the effects of BLyS on B cell physiology have shown that it also regulates immunoglobulin secretion. To determine the relevance of the BLyS receptor-ligand system in Waldenstrom’s macroglobulinemia, we examined malignant B cells from 5 patients with Waldenstrom’s macroglobulinemia for their ability to bind soluble BLyS and for the expression of the known BLyS receptors, TACI, BAFF-R, or BCMA. The malignant B cells were found to bind BLyS and express BAFF-R and TACI. BCMA expression was undetectable. We then determined the expression of BLyS in bone marrow specimens from 5 patients with Waldenstrom’s macroglobulinemia by immunohistochemistry and compared it to the expression in 5 normal bone marrow specimens. The lymphoplasmacytic cell infiltrate in the bone marrow of patients with Waldenstrom’s macroglobulinemia showed significantly increased BLyS expression. We further determined the serum BLyS levels by ELISA in stored serum specimens from patients with Waldenstrom’s macroglobulinemia (n=20), and compared them to serum BLyS levels in other patients with lymphoplasmacytic lymphoma without elevated immunoglobulin levels (n=10) and to serum levels in normal controls (n=50). Serum BLyS levels in Waldenstrom’s patients (mean: 49.6ng/ml) as well as those in patients with lymphoplasmacytic lymphoma (mean; 46.7ng/ml) were significantly higher than normal controls (mean 12.6ng/ml). In conclusion, we have demonstrated that malignant B cells from patients with Waldenstrom’s macroglobulinemia express the receptors for BLyS and can bind soluble BLyS. Furthermore, we have found that serum BLyS levels are significantly elevated in patients with Waldenstrom’s macroglobulinemia when compared to controls. Strategies to inhibit BLyS may potentially have significant therapeutic efficacy in Waldenstrom’s macroglobulinemia.